The field of the present invention relates to optical components. In particular, improved surface accuracy of a thin optical component is described herein.
The surface flatness of a transmissive or reflective optical component (or more precisely, the surface accuracy of a nominally flat optical component) is an important parameter in optics. For purposes of the present disclosure and appended claims, surface accuracy is defined as the maximum deviation of a surface of an optical component from its idealized nominal surface shape, whether that nominal shape is flat, spherical, spheroidal, ellipsoidal, paraboloidal, hyperboloidal, cylindrical (including circular, elliptical, parabolic, or hyperbolic), or other suitable or desirable shape. Surface accuracy is typically reported as a fractional wavelength (e.g., λ/4 or λ/10), because it is often measured relative to interference fringes produced by nearly monochromatic light, e.g., from an atomic lamp or a laser. Surface accuracy reported in that way only has meaning relative to the wavelength used to characterize the surface; 632.8 nm light from a HeNe laser is often used, but any suitable wavelength can be employed. Alternatively, surface accuracy can be expressed as an absolute deviation (e.g., 100 nm or 200 nm).
Surface accuracy should be distinguished from surface roughness or surface quality. Roughly speaking, surface accuracy characterizes the component over longer distances than surface quality or roughness. Surface accuracy mostly affects how a propagating optical signal deviates from its designed behavior after interacting with the component, due to, e.g., wavefront distortion induced by the component. Surface quality or roughness mainly manifests itself as optical loss or scatter from the component. Methods and articles disclosed herein primarily address the issue of surface accuracy of a relatively thin optical component. Wavefront distortion due to inadequate surface accuracy can cause undesirable system performance issues. For example, efficiently coupling an optical signal into a single mode optical fiber requires a near-diffraction-limited wavefront, so as to enable the optical signal to be focused to an aberration-free focal spot that closely matches the mode properties of the fiber. Wavefront distortion induced by a non-ideal optical component can lead to undesirably or unacceptably large insertion loss penalties arising from mismatch between the resulting aberrated focused beam and the fiber mode.
It is often the case that, to maintain a needed or desired degree of surface accuracy, a typical optical component comprises a relatively thick substrate that has sufficient rigidity to resist bending, bowing, or warping during fabrication and use. Such bending, bowing, or warping would induce deviations from the component's nominal shape, resulting in generally undesirable wavefront distortion when used. Avoiding such bending, bowing, or warping is substantially more difficult when using a relatively thin, sheet-like optical component. Examples of such thin optical components are those fabricated from silicon or quartz wafers using, e.g., spatially selective material processing techniques such as those employed in the semiconductor industry. However, such fabrication methods offer cost competitiveness and fabrication accuracy unparalleled by traditional optics manufacturing methods, so that ensuring surface accuracy of such thin components is an important goal. Tools and methods for improving surface accuracy of thin optical components, including those fabricated from wafer materials, to maintain surface accuracy of such component within operationally acceptable specifications, are needed and are disclosed or claimed herein.